In a photovoltaic (PV) conversion system using multicellular solar panels, optimization of power yield under varying conditions of illumination of generally series connected strings of photovoltaic cells, imposes deployment of a way for automatically by-passing strings of series connected cells that are momentarily shadowed or blinded. Momentarily blinded or failed cells do not generate any electricity and are not conductive, thus other series connected illuminated cells may produce a voltage condition across one or several cells of the string that may momentarily be shadowed or practically blinded, which may cause a voltage breakdown and irreversible damage of the shadowed cells. Moreover, one or more faulty or blinded cells of a string may subtract the contribution to the overall power yield of a large number of other illuminated cells of the panel. In large multi-panel installations, a similar requirement of automatic by-passing blinded or faulty conversion units may apply to a whole module of strings or to a single panel of a plurality of panels connected in series, as schematically depicted in FIG. 1.
On another account, in DC power distribution networks including several distinct DC power sources the outputs of which are connected in parallel to a system DC load bus, the individual power converters (which may even be photovoltaic cell panels or other photovoltaic conversion device) may be controlled in a current sharing and/or hot swapping mode by a dedicated control circuit. Each of them normally has a circuit breaker device for interrupting the connection of any faulty DC power source to the DC load bus of the system.
Finally in battery powered systems, it cannot be excluded the possibility that the supply battery polarity be inadvertently reversed. As a safety measure against the risk of damaging the battery powered integrated circuit in case of accidental inversion of the polarity of the battery, a reverse bias protection device is introduced between the battery and the powered integrated circuitry, as schematically depicted in FIG. 2.
A known approach is to install inside the so-called junction box of the PV panel, Schottky diodes connected in parallel to strings of PV cells in series of the multicell panel that are often connected in series or in a series-parallel scheme to the positive and negative terminals of the panel, for providing an alternative path to the flow of electrical current, by-passing the string of PV cells. The current generated by illuminated strings of series connected cells eventually by-passes any string of series connected cells, some or all of which may be shadowed or “blinded”, by flowing through the by-pass Schottky diode of the string of shadowed cells, to permit delivery of output power by the panel that otherwise could be prevented. In these conditions a significantly large power loss occurs in the conducting by-pass diode for as long as the shadowing continues and the diode may heat up considerably during exceptionally long lasting shadowing of the cells of the related string.
WO 2006/079503 A2 discloses the use of a controlled electronic switching device, preferably including a pair of MOSFETs, which is normally nonconductive when the related string of PV cells in series are illuminated and generating an electric current. In case of partial or total shadowing of the string of PV cells, the voltage that is produced at the two terminals of the controlled electronic switching device is exploited to charge a capacitor through a discrete inductive supply circuit. As long as the voltage on the capacitor remains above a certain value, the electronic switching device is kept on, otherwise it is off, allowing the capacitor to be eventually recharged. The charge of the capacitor is controlled by a dedicated commercial voltage regulating IC.
US 2008/0198523-A1 discloses the use of a single MOSFET in place of the by-pass diode, of a discrete inductive supply circuit and of a control and drive circuitry of the by-pass MOSFET. The switching on/off of the MOSFET during a period of shadowing of series connected PV cells of the related string is controlled in a cycling fashion by a timing circuit. Compared to other known approaches, the power loss in the by-pass MOSFET is reduced to a few Watts (about 1W for a current of 15A) depending on the actual duty cycle of the by-pass MOSFET.
US 2010/002349-A1 discloses a monolithic integration of a by-pass power MOSFET controlled by a circuit powered through an integrated charge-pump circuit employing an oscillator and an array of integrated capacitors. Due to leakage, the output voltage of the charge pump slowly decays, therefore the MOSFET is periodically switched off and the accumulated charge restored. During shadowing conditions of PV cells of the related string, the by-pass MOSFET is cyclically switched on/off under the control of a hysteresis comparator circuit that monitors the output voltage of the charge pump circuit.
Notwithstanding the possible advantages over the use of a Schottky diode, there is still a significant power loss during shadowing phases of related PV cells due to the on/off switching of the MOSFET, in practice tied to the off time phases of the by-pass MOSFET. Proposals for eliminating the need for off-time intervals have the drawback of requiring burdensome charge pump circuits employing numerous relatively large discrete capacitors difficult to embed in a system-in-package device of desirably reduced size.
In terms of utility, it should be clear to those skilled in the art that a current by-pass power device may be deployed for optimizing overall power yield from the single multicellular PV panel as well as for optimizing overall power yield from a multipanel PV generation plant including strings of series connected PV panels or alternative PV conversion units, for eventually by-passing any or all panels or units of an affected string of series connected panels or similar units.
Notwithstanding that the above review of the state of the art refers to the function of a current by-pass power device and refers to utility in the architecture of a PV multi-cell panel generation panels, the by-pass device may have the same utility even for DC power distribution networks that include a plurality of series connected power sources (e.g. batteries) any of which may accidentally fail or be momentarily deactivated, and in battery powered systems.